Examples of optical fiber types of pressure transducers used for medical purposes can be found in U.S. Pat. No. 5,005,584 to Little. Generally, the optical fiber type of pressure transducer converts sensed pressure variations into a light modulated signal. The device is calibrated so that the light signal is proportional to pressure. An optical pathway is the preferred mode of light transmission from a light source usually disposed within a control unit to the pressure sensor and back to the control unit. This optical pathway is usually provided by optical fibers or leads. Such leads may be in the configuration of a single fiber which serves to both send and receive light signals to and from the transducer, in the form of two optical fiber leads where one fiber serves to transmit and one to receive light signals, or in the form of a fiber bundle where two or more fibers function together in the manner of a single fiber. The proximal end of fiber optic pathway terminates at a connection site with the control unit and its distal end terminates within a pressure transducer assembly. Light exits the transmitting fiber or fibers, crosses a gap, and is reflected off a reflective surface into the return fiber or fibers. The optical pathway thus serves as a passage for the transmission of pressure indicating signals from the pressure transducer assembly to the control unit, which in turn provides an output signal to a display device.
The pressure transducer assembly includes a housing that receives the distal end of the optical pathway. A cutout or window is made into the side of the housing. The window is often enveloped by a pressure sensitive member that serves to modulate the intensity of the light transmitted from the pressure transducer assembly back to the control unit on the return side of the optical pathway. One surface of the pressure sensitive member is exposed to the pressure desired to be measured and the opposing surface is exposed to a reference pressure, normally ambient atmospheric pressure inside the housing. The membrane is deflected into the window by the pressure and occuludes a portion of the light transmitted. As the pressure externally changes, the membrane will variably occlude the light and thus modulate the light signal accordingly.
The pressure transducers used by medical practitioners are small and often hand assembled. In some embodiments, the construction of the pressure transducer assembly requires inserting the optical fibers within a cylindrical bore and then affixing them in place. Conventional techniques for holding the fibers in their preselected positions include using a potting or cementing material placed within the housing. While useful, this method of fixing the optical fibers in place necessitates insertion of potting or cementing material into the housing along with the fibers. This extra step and the additional material increases the risk of contamination of the transducer assembly, an undesirable situation where sterility is prized.
Current housing configurations include an aperture having a square or curved configuration with the pressure sensitive membrane engaging the aperture in a covering or enveloping relation thereto. During periods where the sensed pressure is decreasing relative to the reference/ambient atmospheric pressure, the returning, modulated light signal may exhibit hysteresis. The hysteresis has been found to arise from the manner in which the pressure sensitive membrane deforms inwardly into the housing aperture as the pressure to be measured increases. This effect must be accounted for in order for the pressure transducer to provide accurate readings.
It would be desirable to provide an improved method of manufacturing an optical fiber pressure transducer and a transducer that exhibited hysteresis to a smaller degree as presently available transducers.